This invention relates to a method and a system for controlling airflow in a multiple bed desiccant drying system, particularly in a twin tower desiccant dehumidifier, and particularly during a regeneration phase, and during transition periods between phases.
Multiple desiccant bed systems used for drying a synthetic plastic material are known, in which a moisture-laden gas stream is formed as the exit gas from a hopper in which plastic granules are dried by a stream of drying air. During an adsorption phase, the exit gas from the hopper is conducted through one or more drying vessels filled with an adsorption medium, whereby the adsorption medium extracts the moisture from the gas so that the resulting dry gas can be used again as a drying gas for drying plastic granules.
When the adsorption medium in a drying vessel is saturated with moisture, the drying vessel is transferred to a regeneration phase in which heated ambient air is conducted through the adsorption medium which takes up and carries away the moisture which was adsorbed therein. The ambient air used to dry the adsorption medium typically itself contains moisture, which increases the drying time required to regenerate the adsorption medium.
Since the adsorption medium is heated during the regeneration phase by the heated regeneration air, the adsorption medium typically must subsequently be cooled with a cooling air flow prior to a transition to the adsorption phase. If moisture-laden ambient air is used as the cooling air, the adsorption medium will adsorb the moisture therefrom, reducing the efficiency (i.e. dryness) of the regeneration process.
By using a multiple bed system, the drying process can be continued essentially without interruption, by utilizing one or a portion of the beds for adsorption, while simultaneously regenerating other of said beds, by appropriately channeling the process air flow.
Prior art twin tower dehumidifiers typically use two four-way valves to divert the respective air streams to the process and regeneration desiccant towers. This is done by porting the left and right sides of the four way valve to the desiccant towers, and by using the top port for the process air and the bottom port for the regeneration air. In this manner twin tower units are constructed for simple selection of the process airflow to one tower and the regeneration airflow to the other tower. The control of regeneration heating is limited to turning the regeneration fan and heater on.
An alternate method of construction for a twin tower adsorber system is to use a poppet type valve that will divert a central, process air inlet port to the left or right tower, and will then xe2x80x9cbleedxe2x80x9d a fixed airflow for the regeneration of the opposite tower. In this xe2x80x9cbleedxe2x80x9d method of regeneration with a continuous regeneration airflow, the control of the regeneration is limited to applying heat, with no control of the airflow.
Multiple desiccant tower dehumidifiers typically use a rotating xe2x80x9ccarouselxe2x80x9d which holds the desiccant towers. In these systems the control of the regeneration is governed by the rotation of the desiccant system and is limited to the progressive rotation of the towers from one stage to the next. There is no individual control of the regeneration airflow with this type of unit. Heating of the desiccant is normally accomplished by the use of an external heating unit with the regeneration airflow conveying the necessary heat into the desiccant tower.
Conventional multi-tower desiccant bed dehumidifier controls utilize a single point proportional integral derivative (PID) anticipating temperature controller in which a thermocouple at the heater unit outlet measures the result of the control operation and feeds this result back to the controller. The controller uses this feedback to make adjustments to the heater operation by balancing the heater on-off cycles accordingly. Such control systems are sensitive to inlet air temperature variations and thus are subject to the drawback that they are limited in the amount of inlet air temperature disruption they can manage without unacceptable temperature variations appearing at the process air outlet. Experience has shown that an inlet air temperature variation of xc2x110xc2x0 C. (xc2x118 to 20xc2x0 F.) over a few minutes will disrupt such a controller to such an extent that unacceptable temperature fluctuations will occur in the process air stream. Since the inlet air temperature variations of as much as 55xc2x0 C. (100xc2x0 F.) typically occur when the towers of a twin tower desiccant bed dehumidifier are switched between the adsorption and regeneration phases, exchange of the towers results in an interruption of processing due to a loss of process of control and requires a restabilization period before processing can resume.
In commonly owned U.S. Pat. No. 5,926,969 to Crawford et al., the entire disclosure of which is expressly incorporated by reference herein, a system and method of operation is disclosed in which two towers are connected by a 4-valve system. The 4-valve system is controlled such that the process air stream is progressively moved from the saturated bed to the regenerated bed. In that invention the terminal disruption of the dewpoint is minimized by any residual heat that remains in the fresh tower being brought online.
A further improvement is described in commonly owned, prior U.S. patent application No. 09/554,680 to Crawford, the entire disclosure of which is also expressly incorporated herein by reference. This improvement to the twin tower desiccant dehumidifier uses a split air stream from the inlet to the system as the cooling medium. In a typical closed loop process system, this inlet air stream has a much lower water content than the ambient air used for regeneration heating. The benefit of this improvement to the regeneration cooling of the desiccant was developed through use of the relative dry process return air as the desiccant cooling medium. While this is a successful method of operation, it has required the use of closed loop control for the management of the cooling air stream to avoid disruption of the process air quality. This is accomplished by the use of modulating control of the four-way dry air control valves of the dehumidifier with monitoring instruments to assure the proper control of the process. As in most industrial processes, the economics of the process system are under substantial discussion, and a premium is placed on reduction of costs for the equipment. Since the control costs are a major concern, there has remained a need for a different method of air control which would enable a less expensive control system to develop equal or greater process performance.
In previously known drying systems, a problematic issue is disruption of the process air temperature and dew point quality when changing from the saturated tower to the freshly regenerated tower. Furthermore, in previously known drying systems, bringing a heat exchanger online at the appropriate time without undue complexity of valves is a known problem. As an additional issue, previously known drying systems may be contaminated by room air during diverter valve changes. In addition, since the common instrumentation used to determine the dryness of the process air stream may require from 30 to 60 minutes to recover from the exposure to a high intermittent dewpoint, it is difficult to determine and monitor the humidity level of the process air in previously known drying systems.
In view of the above, there is a need for an improved method and a system for controlling airflow in a multiple bed desiccant drying system.
There is also a need for a method and a system which can is be implemented in existing multiple bed desiccant drying systems with a minimum of components, effort, and cost.
There is a particular need for a regeneration air control method and system which can maintain adequate process air temperature and dew point stability while cycling of desiccant towers between adsorption and regeneration phases of operation.
There is also a need for a regeneration air control method and system which shields the desiccant beds against undesired exposure to atmospheric moisture.
These and other needs have been met according to a first aspect of the present invention by providing a method for controlling airflow in a desiccant drying system comprising first and second desiccant beds; a first diverter valve communicating with each of the desiccant beds; a regeneration air inlet and a process air outlet; a second diverter valve communicating with each of said desiccant beds, a process air inlet and a regeneration air outlet; a first regeneration air control valve interposed between the regeneration air inlet and the first diverter valve; a second regeneration air control valve interposed between the second diverter valve and the regeneration air outlet; at least one heater for heating regeneration air prior to passage of the regeneration air through the desiccant bed; a first cooling bleed stream line connected between the process air inlet and the second regeneration air control valve; and a second cooling bleed stream line leading from the first regeneration air control valve back to the process air stream; said method comprising the steps of (a) regenerating a first of one of the beds by moving the first diverter valve to a position in which the first desiccant bed communicates with the regeneration air inlet and the second desiccant bed communicates with the process air outlet, and moving the second diverter valve to a position in which the first desiccant bed communicates with the regeneration air outlet and the second desiccant bed communicates with the process air inlet; and (b) subsequently cooling the first bed by moving the second regeneration air control valve to a position in which a process air bleed stream is passed from the process air inlet through a first cooling bleed stream line, the second regeneration air control valve and the second diverter valve to the first desiccant bed to extract heat from the first desiccant bed, and moving the first regeneration air control valve to a position in which the process air bleed stream after passing through the first bed is passed through the first diverter valve and the first regeneration air control valve and returned to the process air stream.
In accordance with another aspect of the invention, the foregoing needs are satisfied by providing a desiccant drying system, comprising first and second desiccant beds, a first diverter valve communicating with each of the desiccant beds, a regeneration air inlet and a process air outlet; a second diverter valve communicating with each of the desiccant beds, a process air inlet and a regeneration air outlet; a first regeneration air control valve interposed between the regeneration air inlet and the first diverter valve; a second regeneration air control valve interposed between the second diverter valve and the regeneration air outlet; at least one heater for heating regeneration air prior to passage of the regeneration air through the desiccant bed; a first cooling bleed stream line connected between the process air inlet and the second regeneration air control valve; and a second cooling bleed stream line leading from the first regeneration air control valve back to the process air stream; in which the second regeneration air control valve in one position blocks the first cooling bleed stream line and in a second position allows a bleed stream of cooling process air to pass through the first cooling bleed stream line, the second regeneration air control valve and the second diverter valve to a desiccant bed to be cooled; and in which the first regeneration air control valve in one position blocks the second cooling bleed stream line, and in a second position allows the bleed stream of cooling air to pass from the desiccant bed to be cooled through the first diverter valve and the first regeneration air control valve back to the process air stream.
The present invention utilizes a pair of three-way poppet valves at the inlet and the outlet of the regeneration air circuit. This enables relatively dry process air to be diverted through a cooling air loop to cool the hot desiccant at the completion of the desiccant regeneration heating cycle. When the heating period has ended, the regeneration air control valve will shift to block the ambient air access and divert a bleed stream of relatively dry process air through the hot, dry desiccant to effect cooling. The air flow for this bleed is controlled by the size of the port orifice to control the rate of cooling. This method makes use of the process air intake cooling coil heat exchanger or cooling coil, which is commonly found in many higher performance desiccant systems, to lower the temperature of the hot effluent of the freshly purged desiccant bed. The exact volume of cooling air is not critical to the successful operation of the system since the heat exchanger removes the additional energy before the combined air enters the xe2x80x9con linexe2x80x9d desiccant bed. At the end of the regeneration cooling period, the temperature and dew point performance of the purged bed has equalized with the used desiccant bed, and a seamless changeover of the multiple tower, four-way diverter valves can be made without disruption of the downstream process.
By introducing a multi ported valve in the regeneration airflow circuit, the present invention isolates the tower system from the ambient air that would otherwise enter the drying system while the diverter valves are in intermediate or indeterminate positions during tower exchange.
The present invention thus relates to a novel desiccant dehumidifier using a unique method of regeneration airflow control to optimize the cooling of a hot, purged desiccant bed before it is placed in the process flow. By directing a regulated flow of air from the closed loop adsorption system, it can be used to cool the purged desiccant without adversely affecting the process. In this way, the desiccant bed temperatures of the two beds can be equalized before the beds are switched between the adsorption and regeneration phases of operation without exposure of the regenerated desiccant bed to ambient moisture.
The present invention has the added benefit of expediting the cooling process to enable reduction in the size of the adsorption desiccant since the limitations of the purge heating and cooling cycle can be reduced by the use of this improvement. The present invention uses an orifice regulated cooling flow to maximize the rate of cooling without addition of high humidity ambient air as a cooling medium for the desiccant system.
High performance desiccant systems typically include a heat exchanger on the inlet of the desiccant adsorption system to depress the operating temperature of the desiccant system to a range where economical use of a particular sized adsorption device is afforded. An especially preferred construction of the present invention uses this heat exchanger in combination with the basic twin tower desiccant system as the core of the device, with the addition of two simple valves at the inlet and outlet of the regeneration air ports. These valves allow the intake and discharge of room air when the regeneration purge heating is engaged to remove the adsorbed water from the desiccant being regenerated. At the conclusion of the purge heating, these valves will be shifted to close the room air intake and outlet, while then connecting the lower side of the desiccant bed to be cooled with the intake of the current adsorption air heat exchanger. The upper side of the hot desiccant bed is connected to the discharge of the adsorption air blower, thereby providing a positive flow of dry air to the top of the hot desiccant bed which, after flowing through and cooling the regenerated desiccant bed, can be returned to the process air stream, e.g. by being recycled to the inlet of the process air heat exchanger. In this way, the present invention enables the cooling of the hot desiccant using relatively dry air from the closed loop process air stream without detrimentally affecting the dry air discharged from the on-line adsorption desiccant bed.
The regeneration control method and apparatus of the invention are capable of maintaining temperature stabilities of xc2x12xc2x0 C. and dew point variations of less than xc2x110xc2x0 C. at the process air outlet of a multi-tower desiccant bed dehumidifier during cycling of the desiccant towers between the adsorption and regeneration phases. Preferably the temperature stability is maintained at xc2x11xc2x0 C. and the dew point is maintained at xc2x15xc2x0 C.
The present invention may be either incorporated into new, or retrofitted to existing, twin tower desiccant dehumidifiers. The disruptive effects of changing the tower diverter valves of a conventional twin tower desiccant dehumidifier are substantially reduced. This is accomplished through the use of a multi port valve applied to the regeneration air circuit of the dehumidifier system.